You purchased a computer system which cost $50,000 5 years ago. At that time, the system was estimated to have a service life of 5 years with salvage value of $5,000. These estimates are still good. The property has been depreciated according to a 5 year MACRS property class. Now (at the end of year 5 from purchase) you are considering selling the computer at $10,000. What book value should you use in determining the taxable gains?